GENUS STREPTORHYNCHUS. 
63 
I shall endeavor to show, in another place, that the species now refer¬ 
red to this genus may be arranged in three natural groups, though it may 
be questioned whether this limitation can always be determined by the 
exterior characters of specimens. 
In the Report on the Fourth Geological District (1843, p. 266), I de¬ 
scribed, under the Genus Strophomena, three species ( S. bifurcata, S . 
arctostriata and S. pedinacea) which now prove to belong to the Genus 
Streptorhynchus. These determinations and descriptions were made from 
few specimens, but the characters were unlike, and were deemed suffi¬ 
ciently constant to entitle them to specific distinction. Larger collections 
of specimens have enabled me to make more extensive comparisons, and 
I am now convinced that these forms graduate into each other, and even 
take a much wider range than was exhibited in the specimens illustrated. 
I am moreover satisfied that the Strophomena chemmgensis of Conrad is a 
Streptorhynchus, and specifically identical with those just enumerated, 
having precedence in point of time. 
In the Tenth Report on the State Cabinet (1857), I described Orthis 
perversa , which belongs to the Genus Streptorhynchus ; and in the Report 
for 1860, I described Orthisina arctostriata and 0. alternata, both of which 
are of the Genus Streptorhynchus. In the same year, Mr. Billings 
described the Streptorhynchus pandora of the Corniferous limestone; an 
appropriate name, perhaps, when we reflect that this is but another form 
of a species to which all those just mentioned must be referred. 
More extensive collections have shown that it is quite impossible to 
accumulate any considerable number of specimens of any one of these 
types, without encountering variations which lead to other forms lying 
intermediate to that one and the other most nearly allied form, until 
finally it becomes impracticable to draw lines of specific distinction 
between them. 
To begin with the oldest form at present included in this group, the 
S. pandora , which occurs in the Schoharie grit and Corniferous limestone, 
we have usually, but not always, a symmetrical form, differing but little 
(if at all) from S. woolworthana of the Lower Helderberg group. We are 
able to trace this form in the Hamilton group, where, although rarely 
[ Paleontology IV.] 9 
